A standard electrical sleeve heater has an inner tube centered on an axis, an electrical heater coil wound around the inner tube, and an outer tube engaged around the heater coil and inner tube. The inner tube is typically formed of a good heat conductor such as copper or brass and the outer tube of a poor heat conductor such as stainless steel. The heater coil is a resistance wire extending in a ceramic powder (MgO) inside a metallic sleeve and has ends extending out one end of the heater.
In order that such a heater can be used for accurate temperature control, it is necessary to provide it or the part being heated with a thermal sensor probe. A typical such sensor probe comprises a pair of constantan wires whose inner ends are connected to leads leading to the control circuitry and whose outer ends are fused together. These wires extend through a thin-walled sleeve of heat conductive material and can produce a millivolt output that is directly proportional to the temperature at the fused ends. The control circuit energizes the heater coil in accordance with the temperature detected by the sensor probe.
Accommodating the thin sensor probe in the sleeve heater is very difficult. There is little room between the coil and the inner tube and often no space at all between the inner tube and the object it is fitted over. If the sensor probe is mounted in place in such a heater it is invariably permanently fixed in position. Thus the critical temperature-sensing end of the sensor probe is often positioned in a disadvantageous location that is not critical to the operation of the system.